The present invention relates to interposed sensors having measuring elements paralleled forcewise, which are fitted in machine parts transmitting force and moment. A principal requirement for such sensors is that they should take the form of disks or plates as thin as possible, so that they alter the installation conditions as little as possible.
The chief requirement for arrangements of this kind is that the subdivision of the total force transmitted through the plate into a smaller measuring force to the force sensors and a supplementary force bypassed must be constant in time.
A sensor of this kind is shown in DE 34 40 670 C2. Glued in these measuring plates are commercial force measuring elements. Their very critical overall heights compared with the measuring plate surface must be adjusted exactly to within a few microns of adapter plates and thrust washers. One consequence of this series connection of a number of disks, of which there are usually already five in the commercial force measuring elements, is great uncertainty in the force transmission by paralleling these elements. This is because eight disks connected in series, for example, with the contact surfaces have ten air gap layers, which according to their preloading have different elasticity ratios.
As a result, the individual force measuring elements in a measuring plate of this kind will give different force signals on account of the ten air gaps, because the size of these gaps is difficult to control. Moreover owing to the fitting of commercial force measuring elements and the necessary adapter and thrust washers, the plate thickness cannot be reduced below a minimum of 10 to 12 mm.
The purpose of the invention is to provide force measuring elements and measuring plates with significantly fewer air gaps in the force measuring arrangement, requiring no additional adapter plates and thrust washers, and being much thinner. This object is attained by the measuring elements consisting of disks having not more than five air gap layers including the force introduction surfaces, whereby the measuring elements are welded in under high mechanical preload, by the sensor surface being ground to ensure flatness, and by the measuring elements forming an assembly unit together with the charge amplifier arrangement. With this combination according to the invention, new installation and monitoring are possible at economical costs, with unified and constant signal conditions of the individual force measuring elements.
The invention relates primarily to piezoelectric force measuring arrangements operating together with charge amplifiers. Quartz (SiO.sub.2) is usually employed as piezo material, allowing measurements lasting up to 15 minutes with only 1 to 2% signal loss, so that satisfactory static calibration is possible. The use of piezo-ceramics may be advisable in certain cases where stronger signals are needed and quasistatic measuring is not important. Nevertheless because quartz plates can be loaded both in compression and shear, depending on the crystal cut, quartz has proved to be the ideal material for multicomponent dynamometry.
The invention can, however, make use of other known sensor techniques. Thus the application of thin and thick film processes and possibly capacitive principles also is conceivable. However, only piezoelectrics makes possible the necessary rigidity against a solid metal plate arranged in parallel.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.